I. Field of the Invention
The present invention relates generally to power inverters and, more particularly, to a power inverter having a switching power supply.
II. Description of Related Art
In a hybrid electric vehicle (HEV) an electric motor is utilized to assist the engine to provide more traction and also to regenerate power to charge the battery. For both driving and regenerating, a power inverter is utilized to complete the conversion between AC and DC.
The power inverter constitutes the main electronic module in an HEV system. In such a system, a 12-volt source is the major power source voltage level for most of the automotive controller electronics. However, it is necessary to convert the 12-volt power source to other voltage levels in order to accommodate the other components of the HEV system. For example, typically, a 3.3-volt source is required for microcontrollers used in the HEV system while a 5-volt source is required for data acquisition. Furthermore, some pre-drivers require even different voltages, such as 15 volts for position sensing like resolver circuit.
In order to obtain the different voltage sources required for the HEV system, it has been the previous practice to utilize switching power supplies to convert the main 12-volt source to the other required voltage sources. Furthermore, power inverters for HEV systems typically are constructed on a multi-layer printed circuit board.
With reference then to FIG. 1, a typical prior art printed circuit board is diagrammatically illustrated. The circuit board 20 includes a 12-volt or main power layer or plane 22 on which one or more control circuits 24, illustrated only diagrammatically, are mounted and powered by the 12-volt power plane 22. The same power layer for the printed circuit board 20 also includes a 5-volt power plane 28, a 3-volt power plane 30, and a 15-volt power plane 32. The power planes 22, 28, 30 and 32 are physically separated from each other on the power layer for the printed circuit board 20 by removal of conductive material in the areas 36.
A first switching power supply 40 then converts the 12-volt source in the power plane 22 to 5 volts for the 5-volt power plane 28 utilizing a standard transformer 42. Similarly, a switching power supply 44 and transformer 46 converts the 12-volt power plane 22 to 3.3 volts for the power plane 30 and, similarly, a third switching power supply 48 and transformer 50 up converts the 12 volts in the 12-volt power plane 22 to 15 volts for the power plane 32.
One disadvantage of the previously known power inverters for HEV systems, however, is that the switching power supplies 40, 44 and 48 generate high amounts of electromagnetic interference (EMI) noise. Such EMI may adversely affect the operation not only of the control circuits 24, but also external circuits 52 which are electrically connected to the power inverter, as well as signal lines 54 also connected to the power inverter through conduction and affect vehicle radio through radiation too. Such EMI can not only cause malfunction of the circuitry for the HEV system, but may also result in incorrect data acquisition, radio static and even possibly compromised vehicle efficiency.
With reference now to FIG. 2, one previously known method of reducing the EMI transmitted from the switching power supplies 40, 44 and 48 to the control circuits 24, external circuits 52 and signal lines 54 has been to provide a bottleneck between the main power plane 22 and the various switching power supplies 40, 44 and 48. Such a bottleneck is formed by removing additional portions of the main power layer 22 so that only relatively small openings 56, 58 and 60 are provided between the main 12-volt power layer 22 and the power supplies 40, 44 and 48. In each case, however, a relatively small portion 62 of the main power layer 22 remains intact to maintain the voltage on the portion of the main power plane 22 on which the control circuits 24 are mounted and the other portions of the main power layer 22 on which the switching power supplies are mounted at the same DC voltage.
While the bottlenecks 62 are effective in reducing the transmission of EMI from the power switching supplies 40, 44 and 48, some EMI necessarily passes through each bottleneck 62 and can adversely affect the control circuits 24 as well as the external circuits 52 and signal lines 54.